Numerous pathological conditions alter the morphology of the ECG signal. For example, cardiac ischemia or an infarct can be diagnosed by observing changes in the ST segment using skin electrodes for acquiring a 12-lead surface ECG. Changes in the R-wave morphology may be indicative of conduction disorders such as bundle branch block. In a clinic, ECG morphology changes can be detected under controlled conditions for diagnosing a change in the cardiac health of the patient.
Implantable and external cardiac monitoring devices can provide ambulatory monitoring of a patient's ECG signal using subcutaneous electrodes or skin electrodes. This ECG data can be used to detect cardiac events that may not occur during an office visit. Data may be transmitted to a programmer or via a remote patient monitoring network to a clinic for review by a clinician. Such ambulatory monitoring of the ECG is useful for detecting arrhythmias based on changes in heart rate. The R-wave of a subcutaneous ECG signal is relatively high amplitude and can be sensed reliably for determining a heart rate.
Detection of morphology changes of a subcutaneous ECG signal is more challenging than R-wave sensing. Bipolar electrodes incorporated on the housing a subcutaneous device are spaced much closer together than 12-lead surface ECG electrodes. This spacing makes the subcutaneous ECG signal morphology prone to changes caused by postural changes of the patient. Even surface ECG monitoring using an ambulatory device may be prone to postural effects since the skin electrodes may be placed closer together with fewer leads recorded than during a 12-lead ECG evaluation. A 12-lead surface ECG is generally acquired when the patient is in a controlled posture the various leads will not all be affected by posture in the same manner allowing accurate diagnoses to still be made. Implantable devices that utilize intracardiac electrodes, such as pacemakers, are less prone to the affects of posture changes since the electrodes are positioned in and sense directly in the heart. The resulting EGM signals are higher in amplitude than the subcutaneous ECG signals. It is desirable, however, to provide ambulatory monitoring of the subcutaneous ECG (or surface ECG) for detecting ECG signal morphology changes associated with various cardiac conditions.